Why are group texts billed as picture/video?

I was shocked on my last bill to be billed for sending 14 picture/video messages, when I was only aware of sending two. Looking at the download detail from My Verizon, I sent a regular text as a Group MMS to 12 people, and it sent them through the "vzwpix.com" and "icmms1.sun5.lightsurf.net" servers. Why would this be billed as a video message instead of as a text message?